(1) Field of the Invention
The present invention relates to spin coating apparatus for use in applying a photoresist or other coating solution in film form to surfaces of substrates such as glass substrates for liquid crystal displays, semiconductor wafers or mask substrates in the manufacture of semiconductor devices.
(2) Description of the Related Art
The spin coating apparatus applies a coating solution to a spinning substrate, and the solution is spread over the substrate by the centrifugal force. In the case of a square substrate, the resulting film tends to be thicker in the corners than in other parts of the substrate.
This is because the corners of a square substrate in a spin cut through ambient air or gas, and are exposed to the air more than other parts of the substrate. As a result, evaporation of the solvent portion of the coating solution is promoted in the corners to increase viscosity of the solution and diminish the spreading effect of the centrifugal force. In the case of a round substrate, the larger the substrate is, the greater is the difference in circumferential speed between a radially outward position and a radially inward position of the substrate. Thus, the tendency to increase the film thickness in peripheral regions is not negligible.
Spin coating apparatus devised to solve the problem encountered with square or large substrates are disclosed in (1) Japanese Patent Publication (Examined) No. 1982-48980, (2) Japanese Patent Publication (Examined) No. 1983-4588, and (3) Japanese Patent Publication Kokai (Unexamined) No. 1989-135565. Such a known apparatus includes a rotary container surrounding a substrate placed in a horizontal posture on a rotary table. The rotary container is rotatable synchronously with the substrate to which a coating solution is applied and spread by the centrifugal force to form a thin film on an upper surface of the substrate.
In the apparatus disclosed in the above three publications, the substrate is invariably enclosed so as to be gastight or approximately gastight in and spun with the container. The spinning substrate is maintained in an atmosphere of the solvent evaporating from the coating solution. In this way, partial gasification of the solvent in the corners of a square substrate, for example, is suppressed. This produces a certain effect of avoiding the film being formed thicker in the corners or peripheries than in inward regions of the substrate as a result of contact with the atmosphere inside the container.
However, the conventional apparatus noted above have the following disadvantages:
(1) In the apparatus disclosed in the Japanese publications (1) and (2) above, because the substrate is enclosed gastight in the rotary container rotatable synchronously with the substrate, superfluous part of the coating solution accumulates in the rotary container. Therefore, an operation must be carried out to remove the superfluous coating solution from the rotary container. For this purpose, the spin coating process may have to be discontinued temporarily, which presents difficulties in successively processing a large number of substrates through the spin coating treatment.
(2) The apparatus in the Japanese publication (3) noted above overcomes the disadvantage relating to continuity of the spin coating process by providing pores or tubes in the bottom of the rotary container for draining superfluous part of the coating solution. However, if the pores or tubes have too small an opening area, the superfluous coating solution is removed only insufficiently, thereby increasing the quantity of mist in the container. Conversely, if the opening area is too large, the solvent gas becomes unevenly distributed in the atmosphere inside the container. It is thus difficult to obtain a uniform film thickness from the spin coating process.
(3) In the apparatus disclosed in the three Japanese publications, the rotary container has a hermetic sealing or approximately gastight construction in order to secure, around the substrate, a uniform atmosphere of the solvent evaporating from the coating solution. Consequently, mist of the coating solution generated with the spin (the coating solution scattering from the substrate and breaking into droplets which float in the air) remains adrift in the rotary container. This means that the substrate lies in a space filled with mist of the coating solution. It gives rise to the problem that the upper and lower surfaces of the substrate are contaminated by the mist. An additional process of cleaning the lower surface of the substrate following application of the coating solution would not only result in an increase in the number of manufacturing steps to lower productivity, but poses a different problem of requiring equipment and running costs for the cleaning process.
(4) The problem of the mist of the coating solution contaminating the substrate does not arise with open type spin coating apparatus that carry out a coating treatment while an area upwardly of a substrate is left open, since the substrate is not hermetically sealed or enclosed approximately gastight in the rotary container. However, as noted hereinbefore, a uniform film thickness cannot be obtained owing to gasification of the solvent in the coating solution which is promoted by the peripheries of the substrate cutting through the atmosphere.
(5) In the case of a square substrate, superfluous part of the coating solution scattering from mid-positions of linear sides could adhere to undersurfaces in the corners of the substrate when passing under the corners immediately after scattering from the sides. In the case of a semiconductor wafer, superfluous coating solution scattering from a mid-position of an orientation flat could adhere to an undersurface of the boundary between an arcuate portion and the orientation flat when passing under the boundary immediately after scattering from the flat. As exemplified by the scattering superfluous coating solution, the lower surface of the substrate is contaminated also by causes other than the mist of the coating solution noted in the paragraph (3) above. An apparatus devised to prevent the contamination of the lower surface of the substrate is disclosed in Japanese Patent Publication (Examined) No. 1983-19350, for example. This apparatus is an open type spin coating apparatus as noted hereinbefore, and includes a nozzle disposed adjacent a rotary table on which a substrate is supported. The nozzle is used to direct cleaning liquid to the lower surface of the spinning substrate under the coating treatment, thereby preventing the coating solution from adhering to the lower surface of the substrate. However, in the case of closed type spin coating apparatus that applies a coating solution to a substrate placed in a sealed rotary container, it is difficult by reason of the structure to fix a nozzle inside the rotary container. Among spin coating apparatus suited for treating large or square substrates, there is no apparatus yet that effectively prevents the coating solution from drifting to the lower surface of the substrate, though such an apparatus has been desired.
(6) The closed type spin coating apparatuses disclosed in the Japanese publications (1) and (2) have the function to produce a "uniform atmosphere of the solvent evaporating from the coating solution" around a substrate, which is achieved by means of the rotary container surrounding the substrate in a gastight condition and rotatable synchronously with the substrate. This is designed to prevent promotion of partial gasification of the solvent in the coating solution, to thereby realize a uniform film thickness. However, it is difficult to apply the coating solution uniformly if the rotary container is closed incompletely after a substrate is placed therein or if packing or the like is defective.